Recently several peptides capable of translocation into a cell by passing through the cell membrane (Protein Transduction Domain, or PTD peptides) have been identified, and attempts have been made to utilize such PTD peptides widely as carriers to transport a protein, nucleic acid, polymer, or other functional molecule into cells.
Noteworthy examples of previously identified PTD peptides include a peptide originating in the Tat protein of HIV (E. Vives et al., J. Biological Chemistry, 272(25), 16010, (1997)), Penetratin (D. Derossi et al., J. Biological Chemistry, 271(30), 18188 (1996)), and VP22, a herpes simplex virus type 1 tegument protein (G. Elliot et al., Cell, 88, 223, (1997)).
The most well-known among the PTD peptides is the Tat peptide, and U.S. Pat. No. 5,652,122 discloses that intracellular transport of β-galactosidase or horseradish peroxidase was accomplished using the Tat peptide. In addition, Zhao et al. have reported performing intracellular imaging by MRI using a conjugate of the Tat peptide and superparamagnetic iron oxide nanoparticles (M. Zhao et al., Biconjugate Chem., 13, 840, (2002)).
Furthermore, Mukai et al. have reported that “YM-3,” which exhibits cellular translocation capability approximately three times higher than that of the Tat peptide, was discovered during research to enhance the function of the Tat peptide (Y. Mukai et al., Biol. Pharm. Bull. 29(8), 1570, (2006)).
On the other hand, we previously identified C45D18, a peptide of 27 amino acid residues represented by SEQ ID NO: 16 (D T W A G V E A I I R I L Q Q L L F I H F R I G C R H) that has stronger translocation capability than a protein originating in Vpr, one of the accessory genes of HIV-1, and we have reported that conjugates of C45D18 and the aforementioned enzymatic proteins are peptides that can translocate not only into a cell, but also into a nucleus, and that the translocation capability thereof is greater than that of the Tat peptide (T. Taguchi et al., Biochem. Biophys. Res. Comm., 320, 18, (2004)). However, that document did not disclose the amino acid sequence necessary for achieving that translocation capability.
Thus several PTD proteins have been identified and modified, and attempts have been made to utilize them widely as a DDS (Drug Delivery System) carrier and the like, but they have not reached practical application on an industrial scale because the cellular translocation capability itself is still not fully understood.